The invention relates to a method of controlling development or etch processes by measuring radiation intensity changes caused by the workpiece during processing.
During the manufacture of integrated circuits, semi-conductor wafers are covered with a radiation sensitive resist layer. Subsequently an image pattern which, in most cases, is very complicated and very finely structured is applied to the resist layer by exposure of the resist layer through masks. Because of the ever increasing packing densities, and since the electrical properties of the integrated circuits making up the final product are, to a considerable degree, dependent on the accurate observance of predetermined line widths and line spacings of the transferred light patterns, the accuracy of the masks employed and the parameters to be observed during exposure are subject to the strictest requirements. It has been found that the very narrow tolerances frequently encountered during the transfer of the light patterns deteriorate in many cases as a result of the development and subsequent etch processes to such an extent that only a small percentage of the integrated circuits manufactured in accordance with this method are serviceable. The deterioration of the tolerances occurring during development or etching is amongst others attributable to the fact that even when precise illumination is used the illumination intensity in the peripheral zone of an exposed area, for example, of a line-shaped area, is, as a rule, substantially lower than in the center. As a consequence, the illumination intensity as a function of the distance from the line center does not follow a rectangular curve but decreases gradually towards the line edges, so that the width of the areas bared by development or etching is largely dependent upon the duration of the development or etch process. Even with an almost rectangular shape of the exposure profiles it is possible, in certain circumstances, to considerably extend the bared areas beyond the exposed areas by what is known as "overetching". The strict requirements to be met with regard to the accuracy of the developed or etched patterns are rendered even stricter by the fact that the manufacture of integrated circuits necessitates a number of exposures in successive process steps.
The development of the exposed photoresist layer is generally controlled by strictly observing a development time predetermined in a series of tests. As in addition to the time, a number of further parameters, such as concentration, purity and temperature of the developer, as well as the composition, purity and thickness of the resist layer, and, above all, the illumination intensity during the exposure of the photoresist layer considerably influence the thickness of the bared lines, even the most accurate process monitoring means will not safely preclude errors.
Therefore, in recent years methods of controlling etch processes have been used to control development processes. It has been found, however, that the methods used to monitor and control etch processes have a number of disadvantages rendering them in many cases unsuitable for the very strict requirements to be observed during the control of etch and development processes. Thus, for example, "IBM Technical Disclosure Bulletin", Vol. 18, No. 6, November 1975, pp. 1867 to 1870, describes a method whereby the reflection of a light beam obliquely incident upon the workpiece is measured and used as a criterion to terminate a development or etch process. This method can only be used to determine the first break-through in an etched layer or the time after which the width of said layer ceases to change but not to determine whether a particular width exists. The same applies to the arrangement described in "IBM Technical Disclosure Bulletin", Vol. 15, No. 11, April 1973, pp. 3532 and 3533, wherein the light penetrating the workpiece, rather than the reflected light, is used to indicate the termination of the etch process.